Where is the 'end station' sighted?
Is it in a hot environment like an airing cupboard or boiler room, if it is, that's why the battery has given up so quickly, batts.don't like heat.
Site it in the cellar or garage the battery will more than likely last longer than 5yrs.
Checking a battery to see 'how many volts' it has is only part of the correct way to test one, on its own, VERY UNRELIABLE.
acorn
June 2011